The present invention relates to a Raster Output Scanner (ROS) and, more particularly, to a ROS having a compact optical system design.
Printing systems utilizing lasers to reproduce information are well known in the art. Typically, such systems include a laser for generating a laser beam, a modulator, such as an acousto-optic modulator or laser diode driver electronics, for modulating the laser beam in accordance with input information to be reproduced, a multifaceted polygon scanner for scanning the modulated laser beam across a medium on a line to line basis, and various optical components to focus and align the laser beam at the medium.
In some printing systems it is required that a cylindrical lens be interposed between the modulated laser beam and the polygon scanner to shape the laser beam to optically correct for motor-polygon induced sagittal beam position errors, commonly referred to as wobble errors. The optical system disclosed in U.S. Pat. No. 3,750,189 illustrates the general utility of such a lens. The cylindrical lens is preferably aligned with the laser beam to redistribute the energy of the modulated laser beam in a manner whereby the energy distribution of the laser beam incident on the recording medium is substantially symmetrical about the start of scan and end of scan positions of the medium, and focused to a small spot. This is particularly important in those systems which utilize a xerographic recording medium.
A compact design for the scanning optics of these prior art type of ROS printers is desirable to make the machine itself as compact as possible and to enable extendability of the same ROS design into many machine architectures.
One well known technique is to introduce folding mirrors to fold the optical path and allow the optical components to be positioned in a more compact area. Another concept disclosed in U.S. Pat. No. 5,142,403, is to replace both a folding mirror and a cylindrical lens in the pre-polygon optics with a single cylindrical mirror. This technique has been found useful for ROS systems which utilize either an underfilled facet design; e.g. the light beam directed against the rotating polygon illuminates only a portion of each facet or in an overfilled facet design; e.g. the light beam directed against the rotating polygon completely illuminates each facet and a portion of adjacent facets. Each design, overfilled and underfilled, has advantages and disadvantages. One of the disadvantages of the overfilled design is that a longer pre-polygon optical path is required to properly image the larger spot at the overfilled facets. A cylindrical lens is required to focus the collimated beam on the facet in the cross scan plane; however, two cylinder lenses may be used in a telephoto configuration which allows for a more compact pre-polygon optical path length. U.S. Pat. No. 4,941,721 illustrates the use of two cylindrical lenses between the beam collimator and the polygon in an overfilled system. According to the present invention, the two cylinder lenses of the prior art are replaced by two cylindrical mirrors so as to approximate a telephoto lens. The first cylindrical mirror must be tilted at a small angle in either the cross scan direction (sagittal plane) or the fast scan direction (tangential plane) in order to eliminate locational interferences between the elements and the beam of light. The second cylindrical mirror may also be tilted the same amount in the opposite direction to aim the beam incident on the polygon facet to be parallel to the plane of the polygon disk. This tilt angle should not be large because abberations may be greater at larger angles. The smaller the mirror, the smaller the angle can be. This design, as will be seen, results in a shortening of the pre-polygon mechanical path length enabling a more compact design which requires no additional elements. Although, technically this idea would work in either an underfilled or an overfilled ROS system, the greatest benefit would result from an overfilled system where the pre-polygon path length is usually much longer than in an underfilled system. More particularly, the present invention relates to a raster output scanner (ROS) imaging system comprising:
a light source for generating a coherent collimated light beam output along an optical path, PA1 a photosensitive image plane, PA1 a rotatable multifaceted polygon interposed in the optical path between the light source and the photosensitive image plane for scanning light beams directed onto the facets of said polygon in a fast scan and slow scan direction across the photosensitive image plane, PA1 a post polygon optical system to focus reflected light beams from said polygon in said fast scan and slow scan directions, and
a pre-polygon optical system including a first cylindrical mirror positioned in the optical path between said light source and said polygon, said first cylindrical mirror reflecting light onto a second cylindrical mirror, the second cylindrical mirror reflecting the beams to said polygon, said first and second cylindrical mirrors creating a focused beam in the cross scan axis of the polygon while maintaining the collimation of the beam in the perpendicular or scanning axis.